Just to add a quick follow-up to my earlier post, one of the reported telltale signs of carbon build-up is a juddering when cold. Mine definitely had it before it went in and, a couple of weeks ago, it came back very slightly.
I used Wurth intake cleaner as an experiment. Although it says a 5.2 litre engine should take two cans, I used half a can to try it out. It took the juddering away straight away.
Considering I've only done 2000 (admittedly gentle) miles since the clean, I think it's an indication of how quickly carbon can form in these engines, so I'm planning on using the cleaner four times a year or so to keep it at bay.
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